1. Field of the Invention
The present invention relates to the magnetic resonance imaging (MRI) technical field, and more particularly to an MRI water-fat separation method, wherein k-space raw measurement data are acquired based on the under-sampling method.
2. Description of the Prior Art
In magnetic resonance imaging (MRI), the body's molecular environment for hydrogen protons in fat tissue is different from that for hydrogen protons in other tissues, which results in a difference in the resonance frequency of the protons. The relaxation time of the hydrogen protons in fat tissue and those in other tissues is also different when they are excited by radio frequency pulses at the same time. When signals are collected at different echo times, the fat tissue and non-fat tissues show different phases and signal strengths.
The Dixon method is known for use to generate pure water proton images in magnetic resonance imaging, and its basic principle is: acquiring the in-phase and out-of-phase echo signals of the water and fat protons respectively, and calculating two signals with different phases to create one pure water proton image and one pure fat proton image respectively. Inhibiting fat in water proton images is thereby achieved.
There are many k-space data collecting methods that can be used in conjunction with the Dixon method in this field, for example, Cartesian trajectory acquisition (sampling), and radial or spiral trajectory sampling. Cartesian trajectory sampling refers to sampling k-space data along a Cartesian path (trajectory), and generating the coordinate space image using Fast Fourier Transform (FFT), and calculating the water and fat images according to the image data acquired in this manner. The single-point Dixon method, two-point Dixon method and three-point and multiple-point Dixon methods are easy and timesaving, but they are sensitive to motion artifacts, and the spin echo sequence is also sensitive to motion artifacts, so there are usually motion artifacts in the images obtained by the Dixon method based on the Cartesian trajectory sampling.
In the radial or spiral trajectory sampling methods, k-space data are sampled along the non-Cartesian trajectory, such as radial trajectory or spiral trajectory. Based on this sampling method, the phase correction and chemical shift correction can be carried out in the image field and k-space to avoid blurred reconstructed images. The advantage of this kind of methods is that the motion introduces fuzziness rather than artifacts into the reconstructed image, which has little impact on identifying the objects in the image, but using the radial or spiral trajectory sampling usually increases the image calculation complexity and takes much more time.
As mentioned above, the Cartesian trajectory sampling method is easy and timesaving, but it is very sensitive to motion such as rigid motion and pulsation. The radial or spiral trajectory sampling methods convert the motion artifact into fuzziness in the reconstructed image, but the calculation is complex and it takes much more time. In short, neither of the two methods above can eliminate rigid motion artifacts.
In Chinese patent application, “A Magnetic Resonance Imaging Method Achieving Water-fat Separation”, filed on the same date with this application, a method of magnetic resonance imaging is disclosed that uses BLADE trajectory to collect the raw measurement data of one in-phase image and two out-of-phase images, and performs phase correction for the raw measurement data of the out-of-phase image using the raw measurement data of the in-phase image when reconstructing the out-of-phase image, thus eliminating the motion artifacts in the water and fat images obtained from calculation.
However, when using the conventional BLADE trajectory acquisition, an in-phase image and two out-of-phase images are first reconstructed through a gridding reconstruction method, and then the water-fat separation calculation is carried out. However, the gridding reconstructing method needs a high sampling rate (usually 100% sampling rate) to eliminate the strip artifacts, so it needs a longer scan time to acquire k-space raw measurement data, which causes the entire imaging to take longer and reduces the efficiency of the MRI device.